Direct access to 2-aryl substituted pyrrolinium salts?for carbon centre based radicals without pyrrolidine-2-ylidene alias cyclic(alkyl)(amino)carbene (CAAC) as a precursor

Article abstract of DOI:10.1039/c8sc05477k

The synthesis of organic radicals is challenging due to their inherent instability. In recent years, cyclic(alkyl)(amino)carbene (CAAC)-derived 2-substituted pyrrolinium salts have been used as synthons for the synthesis of isolable carbon-based radicals. Herein, we report a direct, easy and convenient method for the synthesis of 2-aryl substituted pyrrolinium salts without using CAAC as a precursor. These cations can be reduced to the corresponding radicals. The influence of the aryl substituent at the C-2 position on radical stabilization and dimerization has been investigated. Because of the large scope of our strategy (capability to modulate different substituents at all the C- and N-centres of the pyrrolinium salts), it has the merit to be an extremely effective and productive route for generating carbon-based radicals whose stability as well as reactivity can be varied.

Full text of DOI:10.1039/c8sc05477k

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Sobottka, R. Dolai, A. Maiti, D. Dhara, P. Kalita, R. S. Narayanan, V. Chandrasekhar, B. Sarkar and A. Jana,
Chem. Sci., 2019, DOI: 10.1039/C8SC05477K.
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Direct Access to 2-Aryl Substituted Pyrrolinium Salts for Carbon
Centre Based Radicals without Pyrrolidine-2-Ylidene alias
cyclic(Alkyl)(Amino)Carbene (CAAC) as a Precursor
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
a
a
DOI: 10.1039/x0xx00000x
Debdeep Mandal, Sebastian Sobottka, Ramapada Dolai, Avijit Maity, Debabrata Dhara, Pankaj
c a a,d b
Kalita, Ramakirushnan Suriya Narayanan, Vadapalli Chandrasekhar,* Biprajit Sarkar,* Anukul
www.rsc.org/
a
Jana*
Synthesis of organic radicals is challenging due to their inherent instability. In recent years, cyclic(alkyl)(amino)carbene
(CAAC)-derived 2-substituted pyrrolinium salts have been used as synthons for the synthesis of isolable carbon based
radicals. Herein, we report a direct, easy and convenient method for the synthesis of 2-aryl substituted pyrrolinium salts
without using CAAC as a precursor. These cations can be reduced to the corresponding radicals. The influence of the aryl
substituent at the C-2 position on radical stabilization and dimerization has been investigated. Because of the huge scope
of our strategy (capability to modulate different substituents at all the C- and N-centres of the pyrrolinium salts) it has the
merit to be an extremely effective and productive route for generating carbon-based radicals whose stability as well as
reactivity can be varied.
8
and cation) compounds.
Introduction
Radicals play very crucial roles in various chemical and
1
biological processes, as well as in industrial processes such as
2
in polymer chemistry. Recent reports also suggest their
extensive use as catalysts and reactive intermediates to
3
synthesize various types of molecules. However, because of
their inherent instability and high reactivity most radicals exist
as very short lived species. To understand the nature and
reactivity of radicals formed in various reactions as
intermediates their stabilization and isolation is very crucial.
Efforts in this regard include trapping unstable radicals by
6
a
Scheme 1 Selected chemical structures of CAAC-derived α-carbonyl radical I,
6c
6f
tetrafluoropyridyl radical II
, and propargyl radical III.
So far all the reported CAAC-scaffold-based carbon radicals
such as IV are generated from their corresponding
pyrrolidinium salts, V, derived from CAAC, VI (Scheme 2).
However, a serious limitation and disadvantage of using
CAACs as precursors for generating radicals is their very
limited number, which severely restricts variation of the
substituent scope in the products. To get CAAC-scaffold-
based carbon centre radicals the synthesis of 2-substituted
pyrrolinium salts as immediate precursors is crucial
4
coupling with other stable radicals, or quenching them with
5
suitable
reagents.
In
recent
years,
cyclic(alkyl)(amino)carbenes (CAACs) have been used for the
isolation of different kinds of radicals including those that are
9
6
carbon based (Scheme 1), as well as hetero-atom based
7
systems. In particular, Bertrand et al. have recently reported
the syntheses of CAAC-derived organic mixed valence (radical
(Scheme 2). In other words, if the synthesis of 2-
a.
substituted pyrrolinium salts can be achieved through
Tata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad-
1
0
b.500107, Telangana, India. E-mail: ajana@tifrh.res.in
alternate and more convenient routes, without using
CAAC, this will enrich the library of 2-substituted
pyrrolinium salts as well as the corresponding radicals.
Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin,
Fabeckstraße 34–36,14195, Berlin Germany. E-mail: biprajit.sarkar@fu-berlin.de.
School of Chemical Sciences, National Institute of Science Education and Research,
HBNI, Bhubaneswar-752050, India
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016,
c.
Therefore,
a new methodology for 2-substituted
d.
pyrrolinium salts,
required.
V
with a wide substrate scope is urgently
India. E-mail: vc@iitk.ac.in
†
Electronic Supplementary Information (ESI) available: Experimental details, NMR
H
spectra, X-ray crystallographic data and theoretical details. CCDC 1868053 (
3
),
Me
Et
tBu
H
2
1
868056 (3 ), 1868057 (3 ), 1868055 (3 ) and 1868054 (3 ). For ESI and
crystallographic data in CIF see DOI: 10.1039/x0xx00000x.
This journal is © The Royal Society of Chemistry 20xx
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almost perpendicular to each other (torsion angle: 86.93° for
View Article Online
tBu 12
H
Me
Et
DOI: 10.1039/C8SC05477K
3
, 89.60° for
3
, 87.55° for
3
, and 90° for 3 ).
Scheme
2 Reported strategy and our strategy for the synthesis of 2-aryl
substituted pyrrolinium slats,
V.
Results and discussion
Herein, we have considered a new strategy where 2-
H
substituted pyrrolinium salts,
V, can be generated from a Figure 1 Solid state molecular structure of 3 , thermal ellipsoids at 30%. All hydrogens
secondary ketimine, VII, which has only one α-hydrogen, and a are omitted for clarity. Selected bond lengths (Å) and angles (°): N1‒C1 1.275(3), C1‒C2
1
.508(3), C1‒C5 1.486(3), N1‒C1‒C2 113.34(19).
compound having geminal electrophic centres, VIII. The
secondary ketimine, VII, can be obtained, in three very
convenient steps (Scheme 2), from commercially available
The cyclovoltammogram of 2-aryl substituted pyrrolinium
triflate, 3 , reveals two reduction steps, the first event at
around E1/2 = −1.8 V (vs. FcH/FcH ) which is completely
R
reagents: acid chloride, IX; primary amine,
X; and secondary
+
11
Grignard reagents, R CHMgX, XI
.
In this strategy, there is a
2
reversible on the cyclic voltammetry timescale and indicates
huge scope for variation at all the C-centres as well as at the N-
centre of the pyrrolinium ring. Moreover, the expansion of the
five membered ring is also possible.
R•
the formation of a neutral radical,
3
(Figure 2). The
substituents on the phenyl rings do not have any significant
influence on the redox potentials of these compounds. The
To test our strategy we considered 4-alkylbenzoyl chlorides,
R
=
1
, which are commercially available, as the starting
second reduction step is irreversible and is seen around E1/2
−
2.53 to −2.75 V indicating the formation of 2-e reduced
precursors, keeping in mind that the 4-alkylphenyl substituent
will be at the C-2 position of pyrrolinium salts which can
influence the stability as well as the self-association of the
radicals, which may be generated. Moreover, this choice will
give access to the first non-fluorinated 2-aryl-substituted
pyrrolinium salts and the subsequent radicals derived from
R−
anionic compound,
3
which could not be generated
6c
chemically (Figure 2). Furthermore, the second reduction
step has a profound influence on the reversibility of the first
reduction step even on the timescale of the cyclic voltammetry
experiments (see Figures S17-21 in ESI).
them. For N-substitution we have considered the iPr-group.
R
The reaction of
1
with iPrNH /Et N, PCl , and iPrMgCl
2
3
R
5
sequentially, leads to the imine,
Scheme 2). Subsequently, the reaction of
2
as a colorless liquid
1
2
R
(
2
with nBuLi,
isobutylene oxide and triflic anhydride, sequentially in one pot
leads to the formation of 2-aryl substituted pyrrolinium
R
12
triflates,
3 as white solids (Scheme 3).
R
Scheme 3 Synthesis of 2-aryl substituted pyrrolinium triflate,
3 .
R
All the 2-aryl substituted pyrrolinium triflates,
3
were
characterized by NMR spectroscopy as well as by single-crystal
X-ray measurements (see Figure 1 for the molecular structure
H
Me
Et
R
-
Figure 2 CV of 3 in THF / 0.1 NBu
.
4
PF
6
measured at a GC working electrode at 100 mVs
of
and
reveal that that the pyrrolinium- and the phenyl rings are
3 and Figures S1-S3 for the molecular structures of 3 , 3 ,
1
tBu
3
in ESI). An examination of the molecular structures
2
| Chem. Sci., 2018, 00, 1-5
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In order to chemically access the species that are generated
ARTICLE
View Article Online
DOI: 10.1039/C8SC05477K
H
electrochemically, we carried out the reduction of
3 with KC8
in THF (Scheme 4) which leads to a dark red solution from
which pale yellow crystals could be harvested after keeping
the n-hexane extraction at −35 °C, overnight. Analysis of the
single crystal X-ray diffraction study showed the formation of
H
the dimer,
3
(Figure 3) which must have been formed by a
2
H•
head-to-tail dimerization of the initially formed radical
the case of the Gomberg triphenylmethyl radical, Ph C . The
formation of the radical
3 as in
•
13
3
H•
3
in solution was confirmed by ESR,
H•
tBu•
4 6
Figure 4 EPR spectra for 3 and 3 in 0.1 M NBu PF in THF.
which reveals a signal centered at g = 2.003 (Figure 4).
H
Compound
monomer,
3
3
exists in equilibrium, in solution, with the
2
H•
H
. Also there is the possibility of formation of 3
2
H
by a coupling between the starting cation,
electron reduced compound,
3
and the 2-
H−
3
which has been observed in a
1
2
spectroelectrochemistry study. This is analogous to the
reactivity of the tritylcation which forms adducts with neutral
bases. Both mechanisms were considered for simulating the
cyclic voltammograms and this provided the best fits with the
experimental voltammograms (see Figures S33-37 in ESI).
14
H•
tBu•
Figure 5 Spin densities of 3 (left) and 3
(right) at PBE0/IGLO-III, iso-value 0.01.
H•
Apart from the head-to-tail dimerization of
3 , there is a
possibility of tail-to-tail and head-to-head coupling, the latter
being very unlikely because of steric reasons. Theoretical
calculations reveal that head-to-tail dimerization is the
energetically favourable process. On the other hand, the
formation of tail-to-tail dimer is an endothermic process and is
destabilized compared to the head-to-tail dimer (see Table S15
and Figure S52 in ESI).
Subsequently, we carried out the reduction of other 2-aryl
H
Scheme 3 Synthesis of
3
2
.
substituted pyrrolinium salts with KC (Scheme 4), where the
8
substitution at the 4-position of the aryl group can block the
1
5
Me
Et
dimerization.
Both
3
and
3
afford a mixture of
decomposition products at room temperature although the
Et
reduction of
3 immediately results in a red solution at room
tBu
temperature. In the case of
3
, KC8 reduction produced a
deep red color solution at about −50 °C which remains
sufficiently intense until its work up at room temperature.
Thus, after removing the THF from the reaction mixture we
extracted the residue with dry n-hexane. Keeping this extract
at −80 °C afforded deep red crystals (Scheme 4).
Unfortunately, these crystals were polycrystalline in nature.
Several attempts to grow single crystals in solvents of various
polarities like THF and toluene failed to give suitable single
crystals for X-ray diffraction study.
Me•
Et•
tBu•
Theoretical calculations reveal
3
,
3
, and
3
to be more
12
stable than their corresponding higher energy dimers. This is
H
Figure 3 Molecular structure of 3
2
, thermal ellipsoids at 30% and all hydrogens except
Me•
Et•
the reason why
3
and
3
afford decomposed products
C8-H omitted for clarity. Selected bond lengths (Å) and angles (°): C1‒C5 1.370(4),
C5‒C10 1.450(5), C10‒C9 1.331(4), C9‒C8 1.513(4), C8‒C11 1.603(4), C11‒C15
tBu•
rather than the dimer. Due to the steric bulk
3
is relatively
tBu•
1
.545(4); N1‒C1‒C2 108.4(3), C5‒C1‒N1 124.2(3), C5‒C1‒C2 127.2(3), C9‒C8‒C11 more stable. The formation of
3
insitu in solution has been
1
14.8(2), C7‒C8‒C11 114.9(2), N2‒C11‒C12 102.9(2) (Inset: Yellow crystalline
confirmed by the ESR spectroscopy by the observation of the
signal at g = 2.003 similar to what was seen for
H
compounds of 3
2
).
H•
3
(Figure 4).
The hyperfine couplings to the relevant nuclei are likely not
resolved due to unfavorable hyperfine coupling/line-width
tBu•
ratios. However, for
3
a partial resolution of the hyperfine
couplings is observed in the experimental spectrum. The
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calculated spin densities at the PBE0/IGLO-III level of theory pyrrolinium moieties. As a proof of principle, we dVeiemwoArnticslterOantleinde
reveal a localization of the spin densities at the C-2 and the N-1 the synthesis of 1(N)-isopropyl-2-(4-DRO-pI:h1e0n.1y0l3)9-p/Cy8rSroCl0in54iu77mK
atoms of the CAAC-scaffold with only minor contributions at triflates (R = H, Me, Et, and tBu). These compounds can be
the phenyl substituent at the C2 position (Figure 5).
converted to the corresponding neutral radicals which were
characterized by electrochemical, in-situ UV/vis, and
spectroelectrochemical (ESR) methods as well as with DFT
calculations. In the case of the 2-phenyl substituted radical,
the formation of a head-to-tail dimer was observed as for the
Gomberg triphenylmethyl radical. In view of the above, we
believe that our strategy will be useful for the synthesis of
different CAAC-scaffold based carbon centre radicals.
Currently, we are actively pursuing this strategy to realize new
carbon-centered multi-radical centres including diradicaloids.
Me
Et
tBu
tBu•
Scheme 4 Reduction of
3
,
3
, and
3
(left) and picture of
3
(right).
In order to gain insight into the redox properties and the
electronic structures of the various redox states
(
spectro)electrochemical and (TD-)DFT calculations were
R
performed on the 2-pyrrolinium salts,
3
. In the UV/vis spectra Conflicts of interest
all the four 2-pyrrolinium salts are transparent in the visible
region, as expected for such colorless compounds. However,
the in-situ generated one-electron-reduced neutral forms
There are no conflicts to declare.
H•
tBu•
(measured exemplarily for two compounds)
3
and
3
each
Acknowledgements
display one absorption at around 350 nm and a second broad
absorption at around 485 nm (see Figure S58, S59 and Table
S19 in ESI). The UV/vis spectroelectrochemical measurements
clearly display the reversible nature of the first reduction step,
as seen by the re-generation of the starting spectrum with
quantitative intensities on reversing the potential back to the
starting potential (see Figures S57-60 in ESI). The first
reduction step is thus also seen to be reversible on the
spectroelectrochemical time scale. A full TD-DFT calculation
using B3LYP reproduces the bands of both the cationic and the
neutral radical forms with reasonable accuracy (see Figure S66
in ESI).
On performing a second reduction on the compounds via in-
situ UV/vis spectroelectrochemistry the bands at around 350
and 485 nm that belong to the radical form disappear, and the
spectra become more or less transparent in the visible region
expect for a weak shoulder at 330 or 380 nm (see Table S19 in
ESI). As discussed above for the cyclic voltammetry
measurements, this second reduction step is not reversible,
and switching the potential back to the starting potential does
not lead to the regeneration of the starting spectra.
Furthermore, on scanning the potential back, the spectra
corresponding to the one-electron reduced compounds are
not regenerated either. ESR spectroelectrochemistry at the
second reduction potential shows a disappearance of the
signal corresponding to the radical species, possibly indicating
This work is supported by the TIFR Hyderabad. V.C. is thankful
to the DST for a National J. C. Bose fellowship. The high-
performance computing facilities at ZEDAT, FU-Berlin, are
acknowledged for access to computing resources.
Notes and references
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synthesis of 1(N)- and 2-substituted pyrrolinium salts which
are immediate precursors for the synthesis of CAAC-scaffold
based carbon centre radicals. This strategy allows a large
variation of substrate scope at all the C- and N-centres of the
2
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| Chem. Sci., 2018, 00, 1-5
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TOC Scheme
The new strategy for the synthesis of 2-substituted pyrrolinium salts, an immediate precursor for CAAC-
scaffold based carbon centres is reported, which has huge scope in terms of substrate variation. As a
proof of principle, we have synthesized 1(N)-iPr-2-(4-R-phenyl)-3,3,5,5-tetramethylpyrrolinium triflates
(R = H, Me, Et, tBu) and studied the formation of its corresponding radical by chemically as well as
electrochemically.